A susceptor plate for processing semiconductor substrates (wafers) using plasma enhanced chemical vapor deposition (PECVD) and other similar processes must be in firm contact with an electrical ground to prevent warpage of the susceptor plate. When the susceptor plate ground connection is interrupted, the build-up of plasma energy causes differential thermal heating and warpage of the susceptor plate. When the susceptor warps, its edges generally rise so that the plate forms a bowl shape. The position of a substrate supported on such a warped susceptor no longer meets process criteria, and can cause wafers processed on such a susceptor to be rejected.
Repeated heating and cooling of clamped ground connections during normal substrate processing cycles makes it very difficult to maintain a firm ground connection because repeated expansion and contraction or breakage tends to loosen clamped pieces. The gases used in processing semiconductor wafers are often corrosive and work their way into all nooks and crannies, especially spaces between fasteners and the items being fastened, e.g. the ground connection.
The presence of corrosive gas and the repeated thermal cycling, differential thermal expansion, and relaxation creep of metals at elevated temperatures experienced during processing tends to degrade the electrical ground path, usually passing through an aluminum tube clamped to the back of the susceptor. Such conditions over time reduce and relax the clamping force until the electrical ground connection is no longer viable. This problem is exacerbated by the fine detail and precise assembly which is required of existing susceptor assemblies. In this day and age, the increasing demands for accuracy in semiconductor processing require that susceptor warpage be avoided so that coating and etching of semiconductor wafers be done as precisely and repeatably as possible without the susceptor warpage that occurs when the ground connection is not secure.
An illustration of an existing susceptor assembly in a typical processing chamber will highlight the problem. The general configuration of a vapor deposition processing chamber is shown in FIG. 1 (a susceptor assembly configuration according to the invention is pictured). A processing chamber 20 contains a susceptor assembly 31 supporting a wafer 30. Process gas flows through holes in an electrically biased gas distribution plate 22 towards the face of the wafer (substrate) 30 supported by the susceptor assembly 31. The gas distribution plate 22 is often energized by the use of RF power which causes the processed gas to form a plasma. The susceptor disk 32 in the prior art (as shown in FIGS. 2, 3 and 4) is grounded through an aluminum tube extending through a hollow passage in a ceramic susceptor arm 36. The aluminum tube is then grounded (usually by a ground strap 42) to the usually cool process chamber wall.
During processing, the susceptor disk 32 is heated from its backside (the bottom 28 as shown in FIG. 1) by radiant heat from heating lamps 24 shining through a sealed quartz window 26 and a ceramic backing plate 34. The susceptor temperature reaches to approximately 475 to 500 degrees Celsius.
An example, of such a prior an susceptor assembly is shown in FIGS. 2, 3, and 4. The susceptor assembly includes a aluminum susceptor plate 32 having a susceptor hub 68. The plate 32 is backed by a ceramic plate 34 having Swiss cheese type holes (e.g. see FIG. 5) to selectively control the susceptor plate's exposure to radiant heat from the heat lamps 24. A susceptor hub 68 integral with the susceptor plate 32 includes an alignment/grounding blade 69, a thermocouple receiving hole 59, and several holes for receiving fastening studs 73.
The hub 68 is supported by a ceramic susceptor arm 50. The arm 50 includes a tubular passage to protect and guide an aluminum grounding tube 80. The tube 80 extends through the passage but only a partial tube wall extension 81 extends into the hub end 51 and into contact with the bottom of the end of the susceptor hub blade 69. A thermocouple lead 40 is routed through the tube 80 and terminates with the thermocouple end 41 in the thermocouple receiving hole 59.
The susceptor hub blade 69 is aligned to a blade receiving slot 54 and secured to a web 52 in hub end 51 of the arm 50 by integral intermediate flange nuts 76 (only one is shown in FIG. 4) on the studs 73 which are carefully tightened to a predetermined torque. The end of the hub blade is shaped to match the outside of the tube extension 81. To prevent collapsing the tube extension 81 and pinching the thermocouple lead 40, a hollow half cylinder shaped mandrel 71 is located inside the tube extension 81. A clamping block 72 slips over the stud 73 ends and clamps the tube extension 81 between the mandrel 71 and the end of the of the hub blade 69 as stud nuts 74 are tighten to a specified torque in counterbore openings 75 in the bottom of the block 72. There are counterbores in the top of the block 72 which provide clearance for the middle nut flanges 76 of the studs 73, so the block 72 is secured against the flat bottom of the web 52. FIG. 4 shows the block 72 cut away to show: the extension tube 81 clamping arrangement, the middle nut flange 76 of the right stud 73 (also cut away), and the left stud end and nut 74 in the counterbore 75.
The hub end 51 includes a bottom opening recess 55 to receive a ceramic disk shaped circular cover 56 (FIG. 3) to shield the contents of the hub from direct exposure to radiant heating. The cover 56 is retained in the recess 55 by a ceramic pin 58 placed in two pin receiving holes 57 (FIG. 4) aligned across the recess opening 55.
A ceramic collar 48 surrounds the susceptor hub 68 at the top to assist in protecting the thermocouple and hub from the processing chamber environment. This pin sometimes falls out exposing the hub pieces to extreme temperatures resulting from direct exposure to radiant heating.
An aluminum susceptor arm end support 84 is clamped to the support end of the arm 50 to support it from the lift mechanism 38 (FIG. 2). A ground rope 42 is welded to the support end of the aluminum tube 80 and is routed into contact with the end support 84 on its way to a ground connection on the wall of the processing chamber 20.
The thermocouple lead 40 is routed from a threaded thermocouple receiving hole 59 in the back of the susceptor plate 32 between the mandrel 71 and the clamping block 72, through the tube 80, and through a vacuum seal in the central core of the susceptor lift mechanism 38.
To work properly, the above pieces must be carefully assembled. The assembly or disassembly of a large number of pieces according to a detailed assembly procedure unnecessarily complicates the configuration and increases the chance that an initially created ground connection will not be reliable after many processing cycles. When the susceptor is not grounded, the susceptor temperature builds up unevenly and warpage begins to occur.
When potentially thousands of wafers are processed through one processing chamber having a single susceptor assembly, even small variations in the process conditions resulting from susceptor warpage can create an operating problem. Additional monitoring (quality assurance-inspection) is needed to assure that susceptor warpage does not affect wafers being processed.